1. Field of the Invention
The present invention relates to a particle therapy system for conducting cancer treatment by irradiating tumor volumes with a charged particle beam accelerated by a particle accelerator, and a particle therapy planning apparatus (treatment planning apparatus for particle therapy) for arranging a treatment plan for particle therapy.
2. Description of the Related Art
A particle beam cancer treatment system is a system for conducting cancer treatment by irradiating tumor volumes with a charged particle beam accelerated by a particle accelerator. The charged particle beam is accelerated up to approximately the light speed by a particle accelerator such as a synchrotron accelerator or a cyclotron accelerator and transported to an irradiation nozzle by a beam transport system. The charged particle beam is shaped in the irradiation nozzle to form an irradiation field that is conformal to (i.e., that fits) the shape of the target volume and the shaped charged particle beam is applied to the patient. Methods for shaping the charged particle beam in the irradiation nozzle into the irradiation field include a scatterer irradiation method, a scanning irradiation method, etc. In the scatterer irradiation method, the charged particle beam is enlarged by a scatterer and a necessary part of the enlarged beam is cut out by a collimator and applied to the target volume. In the scanning irradiation method, the charged particle beam transported to the irradiation nozzle by the beam transport system is applied to the target volume while directly scanning the beam with scanning magnets so that the scanned beam fits the target shape. The scanning irradiation method is capable of forming a dose distribution coinciding with the target shape since a thin charged particle beam accelerated by the particle accelerator and transported by the beam transport system is directly scanned by the scanning magnets during the irradiation of the target volume,
As an irradiation method placed between the scatterer irradiation method and the scanning irradiation method, there exists a method called “uniform scanning” as described in V. A. Anferov, “Scan pattern optimization for uniform proton beam scanning”, Med. Phys. 36 (2009) 3560-3567 (hereinafter referred to as “Non-patent Literature 1”) and S. Yonai, et al., “Evaluation of beam wobbling methods for heavy-ion radiotherapy”, Med. Phys. 35 (2008) 927-938 (hereinafter referred to as “Non-patent Literature 2”). The uniform scanning is an irradiation method in which the charged particle beam enlarged by the scatterer is scanned by the scanning magnets during the irradiation so as to form a dose distribution that is uniform in lateral directions. In the uniform scanning, a ridge filter is used for enlarging the dose distribution in the depth direction, that is, for forming an SOBP (Spread Out Bragg Peak). Alternatively, the target volume is partitioned into a lot of layers, the layer to be irradiated is switched by changing the energy of the charged particle beam, and the SOBP is formed by properly adjusting the charged particle beam irradiation amount of each layer so that the dose distribution in the depth direction becomes uniform. In the uniform scanning, a bolus, as a patient-specific device for adjusting the dose distribution to the shape of the under surface of the target volume, may also be used. In order to determine the irradiation field shape in the lateral directions conformal to the target shape in the uniform scanning, a multi-leaf collimator (which automatically shapes the irradiation field) or a patient-specific collimator (prepared by cutting a metal plate to form an aperture in a shape conformal to the target volume by electrical discharge machining, etc.) is used.
While only one collimator aperture shape is used in the uniform scanning, there exists an irradiation method called “conformal layer stacking irradiation” in which the target volume is partitioned into layers and each of the layers is successively irradiated by adjusting the aperture of the multi-leaf collimator to the irradiation field shape of each layer, as described in T. Kanai, et al., “Commissioning of a conformal irradiation system for heavy-ion radiotherapy using a layer-stacking method”, Med. Phys. 33 (2006) 2989-2997 (hereinafter referred to as “Non-patent Literature 3”). In the conformal layer stacking irradiation, the target volume is partitioned into a lot of layers and each of the layers is successively irradiated while scanning the charged particle beam with the scanning magnets so that the lateral dose distribution (dose distribution in the lateral directions) becomes uniform similarly to the uniform scanning. The conformal layer stacking irradiation is capable of forming a dose distribution conformal to the target shape since the dose distribution is adjusted to the shape of the under surface of the target volume by using the bolus (patient-specific device) and then each layer of the target volume is irradiated by adjusting the irradiation field shape in the lateral directions to the lateral shape of the layer by using the multi-leaf collimator. The conformal layer stacking irradiation automatically changes the irradiation field shape from layer to layer by employing the multi-leaf collimator for specifying the irradiation field shape in the lateral directions for the irradiation of each layer. In the conformal layer stacking irradiation, each layer of the layer-partitioned target volume is irradiated with a charged particle beam of constant energy, and the layer to be irradiated is switched by changing the energy of the charged particle beam. The conformal layer stacking irradiation is capable of forming a dose distribution more coinciding with the target shape compared to the scatterer irradiation method and the uniform scanning.
In either the uniform scanning or the conformal layer stacking irradiation, the lateral scan is conducted by scanning the charged particle beam, which has been enlarged by the scatterer, in the lateral directions as explained above. A charged particle beam having a larger beam size compared to that in the scanning irradiation method is scanned in the uniform scanning and the conformal layer stacking irradiation, and thus the irradiation field has to be formed by using a collimator so that the irradiation field shape in the lateral directions fits the target shape. In the uniform scanning, the patient-specific collimator prepared by electrical discharge machining or the multi-leaf collimator automatically shaping its aperture to fit the target shape is used. In the conformal layer stacking irradiation, the multi-leaf collimator is used since the collimator aperture has to be changed for each layer of the layer-partitioned target volume.
In the uniform scanning and the conformal layer stacking irradiation, in order to form a dose distribution uniform in the lateral directions orthogonal to the charged particle beam's propagation direction, the charged particle beam is enlarged with the scatterer and the beam having a larger beam size than in the scanning irradiation method is applied to the target shape while scanning the beam in the lateral directions, as described in the Non-patent Literatures 1-3, Japanese Patent No. 3518270 (hereinafter referred to as “Patent Literature 1”), and M. Komori, et al., “Optimization of Spiral-Wobbler System for Heavy-Ion Radiotherapy”, Jpn. J. Appl. Phys. 43 (2004) 6463-6467 (hereinafter referred to as “Non-patent Literature 4”). As shown in the Patent Literature 1 and the Non-patent Literatures 1-4, there exist various methods for forming the uniform dose distribution by superposing a plurality of dose distributions having the Gaussian distribution shape during the scanning of the charged particle beam having the large beam size by use of the scanning magnets.
In the raster scan shown in the Non-patent Literature 1, the charged particle beam is scanned along a scan path in a rectangular area continuously like the one-stroke drawing (drawing a picture with one stroke of the brush) without turning the beam ON or OFF by the accelerator. An area with a uniform dose distribution can be formed by setting the scan line interval of the raster scan at less than 2σ with respect to the beam size σ of the charged particle beam.
In the zigzag scan shown in the Non-patent Literature 2, scan velocities of the charged particle beam in the X and Y directions in the X-Y plane are set independently, and the beam is scanned in a zigzag shape continuously like the one-stroke drawing without turning the beam ON or OFF similarly to the raster scan. In the zigzag scan, an area with a uniform dose distribution can be formed by setting the interval between the scan lines constituting the zigzag shape at less than 2σ (σ: beam size) similarly to the case of the raster scan.
In the circular wobbling shown in the Non-patent Literature 3, the charged particle beam is scanned along a circular scan path, continuously without turning the beam ON or OFF similarly to the raster scan and the zigzag scan. An area with a uniform dose distribution is formed around the center of the circular scan path.
In the spiral wobbling shown in the Non-patent Literature 4, the charged particle beam is scanned along a scan path in a spiral shape continuously without turning the beam ON or OFF. Similarly to the circular wobbling, an area with a uniform dose distribution is formed around the center of the spiral scan path.
In the line scan shown in the Patent Literature 1, the raster scan is combined with the ON/OFF control of the charged particle beam by the accelerator. Specifically, during the raster scan in the X-Y plane, the beam is kept ON in the X-direction scans so as to continuously irradiate the target volume and is kept OFF in the Y-direction scans so as not to irradiate the target volume. In the line scan, an area with a uniform dose distribution is formed by superposing a plurality of continuous X-direction linear dose distributions (like lines extending in the X direction) in the Y direction.
Among the above scanning methods, the raster scan, the zigzag scan, the spiral wobbling and the line scan (in which the beam size of the charged particle beam enlarged by the scatterer is smaller than in the circular wobbling) have some advantages over the circular wobbling, in that the thickness of the scatterer can be decreased, the energy loss of the charged particle beam in the scatterer can be reduced, and consequently, a longer reachable range of the charged particle beam can be achieved. Further, thanks to the small beam size of the charged particle beam, higher beam utilization efficiency can be achieved compared to the circular wobbling. On the other hand, due to the small beam size of the charged particle beam, the scan path tends to be longer than in the circular wobbling and the planar scan time necessary for forming the dose distribution uniform in the lateral directions tends to be longer.